Multijunction photovoltaic device and method of manufacture

ABSTRACT

A multijunction photovoltaic device includes first, second, and third amorphous silicon p-i-n photovoltaic cells in a stacked arrangement. The intrinsic layers of the second and third cells are formed of a-SiGe alloys with differing ratios of Ge such that the bandgap of the intrinsic layers respectively decrease from the first uppermost cell to the third lowermost cell. An interface layer, composed of a doped silicon compound, is disposed between the two cells and has a lower bandgap than the respective n- and p-type adjacent layers of the first and second cells. The interface layer forms an ohmic contact with the one of the adjacent cell layers of the same conductivity type, and a tunnel junction with the other of the adjacent cell layers.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/730,177, filed on Jul. 16, 1991, now U.S. Pat. No.5,246,506, the content of which is relied upon and incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to multijunction photovoltaicdevices and their method of manufacture. More particularly, the presentinvention relates to amorphous silicon multijunction photovoltaicdevices constructed to have improved efficiency and stability.

2. Description of the Related Art

Photovoltaic (PV) devices are used to convert radiation, such as solar,incandescent, or fluorescent radiation, into electrical energy. Thisconversion is achieved as a result of what is known as the photovoltaiceffect. When radiation strikes a photovoltaic device and is absorbed byan active region of the device, pairs of electrons and holes aregenerated. The electrons and holes are separated by an electric fieldbuilt into the device.

In accordance with a known construction of solar cells using amorphoussilicon, the built-in electric field is generated in a structureconsisting of p-type, intrinsic (i-type), and n-type layers (p-i-n) ofhydrogenated amorphous silicon (a-Si:H). In photovoltaic cells havingthis construction, the electron-hole pairs are produced in the intrinsiclayer of the cell when radiation of the appropriate wavelength isabsorbed. The separation of the electrons and holes occurs under theinfluence of the built-in electric field, with the electrons flowingtoward the region of n-type conductivity and the holes flowing towardthe region of p-type conductivity. This flow of electrons and holescreates the photovoltage and photocurrent of the photovoltaic cell.

It is important that the semiconductor material used for making aphotovoltaic device is capable of absorbing and converting to usefulelectrical energy as much of the incident radiation as possible in orderto generate a high yield of electrons and holes. In this regard,amorphous silicon is a desirable material for use in photovoltaicdevices since it is capable of absorbing a high percentage of theincident radiation relative to other materials used in the constructionof photovoltaic cells, such as polycrystalline silicon. In fact,amorphous silicon is capable of absorbing about 40% more of incidentradiation than polycrystalline silicon.

In a p-i-n type amorphous silicon photovoltaic cell as presently knownin the art, the undoped (intrinsic or i-type) layer between the p-typelayer and the n-type layer is proportionally much thicker than thep-type and n-type layers. The intrinsic layer serves to prevent theradiation-generated electrons and holes from recombining before they canbe separated by the built-in electric field. This structure is generallyreferred to as "p-i-n" if the radiation is incident on a p-type layer,and "n-i-p" if the radiation is incident on an n-type layer.

Some of the incident light is also absorbed by the doped layers (thep-layers and the n-layers). Because the carriers generated in theselayers have an extremely short carrier lifetime, they recombine beforethey can be collected. Hence, absorption in the doped layers does notcontribute to the photocurrent of the photovoltaic cell and aminimization of absorption in doped layers enhances the short-circuitcurrent of p-i-n photovoltaic cells. Absorption loss in the p-layer is afunction of the bandgap of the p-layer. Thus, by adjusting the bandgapof the p-layer, the absorption loss in the p-layer can be minimized byincluding in the p-layer a bandgap widening material such as carbon,nitrogen, oxygen or fluorine. For example, the p-layer can be providedas hydrogenated amorphous silicon carbide (a-SiC:H) with p-type doping.

However, the addition of bandgap widening material to the p-layerincreases its resistance. Therefore the amount of bandgap wideningmaterial that is added is usually limited by the amount of resistanceconsidered tolerable in the device.

The n-type layer functions to form a rectifying junction with theintrinsic layer. In order to enhance this function, it is desirable toprovide the n-layer with a high conductivity. However, it is alsodesirable to provide the n-layer with a wide optical bandgap since, asdescribed above, carriers generated therein do not contribute to thephotocurrent of the cell. Unfortunately, as is the case of the p-layer,the addition to the n-layer of any of the bandgap widening elementsdescribed above results in an increase in the resistance of the n-layer.Therefore, the n-layer is typically provided with a concentration of abandgap widening element that is limited by the amount of resistanceconsidered tolerable in the device.

It is desirable to increase the total number of photons of differingenergy and wavelength which are absorbed in order to maximize thephotocurrent output of a photovoltaic device. One technique forincreasing photon absorption, and thereby increase device efficiency, isto provide a multijunction photovoltaic device with two or morephotovoltaic cells arranged in a stacked configuration, i.e., one on topof the other. Such multijunction photovoltaic devices, also known in theart as a tandem junction solar cell, are disclosed in U.S. Pat. No.4,272,641 issued to Hanak (the '641 patent) and U.S. Pat. No. 4,891,074issued to Ovshinsky and Adler, which are incorporated herein byreference. In particular, these patents teach the construction of tandemjunction amorphous silicon solar cells, wherein each cell has the abovedescribed p-i-n structure.

Such multijunction photovoltaic devices consist of a stack of two ormore photovoltaic cells which are both electrically and optically inseries. Typically in such devices, short wavelength light is absorbed ina first, topmost cell, and longer wavelength light is absorbed in secondand, if present, subsequent cells. The first, second and subsequentphotovoltaic cells of the multijunction device preferably respectivelyhave successively narrower optical bandgaps in order to efficientlyabsorb solar radiation.

In order for such multijunction p-i-n photovoltaic devices to operate atmaximum efficiency, current must flow unimpeded from each photovoltaiccell to the next adjacent cell in the stack of cells. However, thenature of the multijunction p-i-n photovoltaic device, i.e., p-i-n-p-i-n. . . , results in an n-p junction occurring at each interface betweenadjacent p-i-n cells and therefore in series electrically with thoseadjacent cells. Disadvantageously, each of these n-p junctionsrepresents a diode having a polarity opposite to that of thephotovoltage generated by each of the adjacent photovoltaic cells. Then-p junctions are non-linear elements that oppose the flow ofphotocurrent and thereby impose a substantial power loss on the device.

FIG. 1 illustrates a plot of current vs. voltage (IV) of a multijunctionp-i-n photovoltaic device. In particular, curve 100 (broken line)represents the IV characteristic for such a photovoltaic device in whichno steps have been taken to overcome the adverse effect of the n-pjunctions at the interfaces between adjacent cells. As illustrated bycurve 100, an inflection occurs in the region where the photocurrent ofthe device changes direction. Such an inflection represents anundesirable increase in the series resistance of the device due to then-p junction. This aspect of the IV curve, characteristic of the n-pjunction, limits the amount of photocurrent that can be conducted by thephotovoltaic device, and therefore lowers the fill factor and powergeneration capability of the device. As used herein, the fill factor ofa photovoltaic device is the ratio V_(mp) I_(mp) /I_(L) V_(OC), whereV_(mp) and I_(mp) are respectively the voltage and current at maximumpower delivery of the device, and V_(OC) and I_(L) are respectively themaximum voltage and current achievable in the device.

A solution to the above described problem caused by the n-p junctions isto modify the structure of the multijunction device so that the junctionoccurring between each pair of adjacent cells performs like a tunneljunction (i.e., a recombination junction). One known method for creatinga tunnel junction between adjacent solar cells of a multijunctionphotovoltaic device constructed from crystalline semiconductormaterials, such as silicon, is to heavily dope the respective n- andp-layers of the n-p junction formed by the adjacent cells. However, thismethod for creating a tunnel junction cannot readily be applied to theabove described multijunction p-i-n devices because amorphous silicon isnot easily doped to yield a highly conducting film. Such difficulty inachieving suitably high conductivity is particularly the case with widebandgap alloys such as hydrogenated amorphous silicon carbide (a-SiC:H)and hydrogenated amorphous silicon nitride (a-SiN:H) which are preferredmaterials for constructing the p- and n-type layers of amorphous siliconp-i-n photovoltaic devices since, as described above, their use tends tomaximize the optical transmissivity of each photovoltaic cell of themultijunction device. As a result, an attempt to highly dope the p- andn-layers of an amorphous silicon multijunction p-i-n device constructedwith wide bandgap alloys does not achieve a desirable tunnel junctioncharacteristic at the n-p junction between adjacent cells.

A method for creating a tunnel junction between adjacent solar cells ofan amorphous silicon multijunction p-i-n device is disclosed in theabove-incorporated U.S. Pat. No. 4,272,641. There, an additional tunneljunction layer is disposed between adjacent p-i-n cells, such layerbeing provided as a cermet incorporating a metal, or as a thin metallayer and a cermet, hereinafter the "metallic layer." While the metalliclayer may function in conjunction with the adjacent cell layers toreduce the above described inflection in the IV curve 100 of the device(FIG. 1), the provision of the extra metallic layer substantiallyinhibits the manufacturing process. In order to manufacture an amorphoussilicon multijunction photovoltaic device using such a metallic layer, afirst photovoltaic cell is formed in a first material deposition system,for example a glow discharge chamber as described in the '641 patent.Next, the device must be removed from the glow discharge chamber andplaced in a second material deposition system where the metallic layeris deposited. For example, the '641 patent describes deposition of themetallic layer by a sputtering process. Then the device must be returnedto the first deposition system where a second photovoltaic cell isformed on the metallic layer. Of course, if the device includes morethan two cells, the process of transferring between the two depositionsystems must be continued. The additional deposition system and the timerequired to manufacture a multijunction device in accordance with such aprocess results in an overall increased cost of the device and reductionin production yield.

Further, while the metallic layer disclosed in the '641 patent isoptically transmissive, it has a lower index of refraction than that ofthe adjacent n- and p-type a-Si:H layers of the cells it is disposedbetween. As a result of the different indexes of refraction, light isundesirably reflected at the interfaces between the metallic layer andthe adjacent a-Si:H layers.

As stated above, the most efficient photovoltaic device will absorb allthe light which impinges on it and convert the energy from the lightinto current. However, the total current produced by a multijunctiondevice is equal to the smallest amount of current generated by one ofits photovoltaic cells. Therefore, the design of multijunction devicesis constricted in the sense that additional photovoltaic cells cannot bearbitrarily added to the device to ensure that all the impinging lightis absorbed and converted into current. Because the first photovoltaiccell of a multijunction device absorbs a large portion of the impinginglight, the second and subsequent photovoltaic cells typically areconstructed with extra thick intrinsic layers in order to maximize thelight absorbed and the current produced by the cell. Further, theintrinsic layers of the second and subsequent cells are typically madethicker to compensate for the absorption of light in the p- and n-typelayers. Therefore, it has been generally recognized that the overallinitial efficiency of such a multijunction device increases with theincrease in thickness of the intrinsic layers of the second andsubsequent cells.

A problem results, however, in that the photodegradability of aphotovoltaic cell increases with increasing thickness of the intrinsiclayer used in the photovoltaic cell. Therefore, there is a trade-offbetween initial efficiency and long term stability of a photovoltaicdevice.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstancesand has as an object to provide an amorphous silicon multijunctionphotovoltaic device having an increased overall efficiency.

A further object of the present invention is to provide an amorphoussilicon multijunction photovoltaic device including a tunnel Junctionbetween adjacent cells which exhibits characteristics that do not limitthe power generation of the device.

Another object of the invention is to provide an amorphous siliconmultijunction photovoltaic device including a tunnel junction betweenadjacent cells which may be formed without the need for additionalfacilities and time to carry out the manufacturing process.

Additional objects and advantages of the invention will be set forth inpart in the description which follows and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and attained bymeans of the instrumentalities and combinations particularly pointed outin the appended claims.

To achieve the objects and in accordance with the purpose of theinvention, as embodied and broadly described herein, a multijunctionphotovoltaic device is provided which comprises a transparent substrate,a transparent electrode formed on the transparent substrate, a firstphotovoltaic cell formed on the transparent electrode, a secondphotovoltaic cell, a third photovoltaic cell, a reflective electrodelayer formed on the third photovoltaic cell, and an interface layersandwiched between the first and second photovoltaic cells and/or thesecond and third photovoltaic cells.

The first photovoltaic cell comprises a p-type layer formed adjacent thetransparent electrode, an intrinsic layer of an amorphous semiconductorcompound including silicon formed on the p-type layer, and an n-typelayer formed on the intrinsic layer.

The second photovoltaic cell comprises a p-type layer, an intrinsiclayer formed on the p-type layer, and being made of an amorphoussemiconductor compound including silicon doped with germanium and havinga bandgap less than the bandgap of the intrinsic layer of the firstphotovoltaic cell, and an n-type layer formed on the intrinsic layer.

The third photovoltaic cell comprises a p-type layer, an intrinsic layerof an amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of the intrinsiclayer of the second photovoltaic cell, and an n-type layer formed on theintrinsic layer.

The interface layer, which is made of a semiconductor compound includingsilicon, is formed on the n-type layer of the first and/or secondphotovoltaic cell, and has a bandgap less than the bandgap of the n-typelayer of the first and/or second photovoltaic cell and less than thebandgap of the second and/or third photovoltaic cell.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

In the drawings,

FIG. 1 is a graph of the current-voltage characteristic of a prior artphotovoltaic device and the current-voltage characteristic of apreferred embodiment of the claimed invention;

FIG. 2 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a first embodiment ofthe present invention;

FIG. 3 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a second embodimentof the present invention;

FIG. 4 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a third embodiment ofthe present invention;

FIG. 5 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a fourth embodimentof the present invention;

FIG. 6 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a fifth embodiment ofthe present invention; and

FIG. 7 is a cross-sectional view of an amorphous silicon multijunctionphotovoltaic device constructed in accordance with a sixth embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

Referring now to FIG. 2, there is shown a multijunction photovoltaicdevice 200 constructed in accordance with a first embodiment of theinvention. Device 200 includes a substrate 202 having an incidentsurface 204 for receiving radiation 206. Substrate 202 preferablycomprises glass or another suitable material which is transparent to theincident radiation. A front conductive contact layer 208 is formed onsubstrate 202. Front conductive layer 208 is preferably an opticallytransparent conductor such as a metal oxide, e.g., tin oxide.

Substrate 202 and front conductive contact layer 208 support twophotovoltaic cells 210 and 212 in a stacked configuration. Photovoltaiccell 210 is comprised of a p-type hydrogenated amorphous silicon carbide(a-SiC:H) layer 220 deposited on front conductive layer 208, anintrinsic hydrogenated amorphous silicon layer 222 formed on p-typelayer 220, and an n-type a-SiC:H layer 224 formed on intrinsic layer222. Photovoltaic cell 212 has substantially the same basic structure asthat of photovoltaic cell 210, in that it is comprised of a p-typea-SiC:H layer 230, an intrinsic hydrogenated amorphous silicon layer232, and an n-type a-SiC:H layer 234. The p- and n-type layers, of cells210 and 212, include carbon concentrations of approximately 20% and 15%,respectively, at least adjacent to the intrinsic layer in each cell. Agrading of the carbon concentration in each of n-type layer 224 andp-type layer 230 is described more fully below with respect to a methodfor constructing device 200. It is noted that while both the p- andn-type layers of cells 210 and 212 include carbon, a multijunctiondevice can be constructed in accordance with the invention withoutincluding a bandgap widening element in all doped layers.

While the p- and n-type layers of cells 210 and 212 can include carbonas a bandgap widening element, intrinsic layers 222 and 232 aresubstantially free of such bandgap widening elements. However, narrowportions of the intrinsic layer, respectively in contact with the p- andn-type layers containing carbon, can include a small, gradedconcentration of carbon. Further, in accordance with a known practice inthe art, intrinsic layers 222 and 232 can include light p-type doping,e.g., a concentration of boron of less than 10¹⁷ cm⁻³, which is known toimprove carrier generation capability of the intrinsic layer. Exemplarythicknesses and doping concentrations of the layers constituting eachcell 210 and 212 are disclosed in the above-incorporated U.S. Pat. No.4,272,641.

Device 200 also includes a back contact layer 236 formed on n-type layer234 and preferably provided as a metal such as aluminum.

An interface layer 240 of relatively low bandgap, high conductivitysemiconductor material is interposed between the n-type a-SiC:H layer224 of photovoltaic cell 210 and the p-type a-SiC:H layer 230 ofphotovoltaic cell 212. In particular, layer 240 has a lower bandgap thaneither of adjacent layers 224 or 230 and can be provided as a p- orn-type semiconductor material that forms an ohmic contact with theadjacent one of layers 224 and 230 of the same conductivity type andthat forms a tunnel junction with the one of layers 224 and 230 of theopposite conductivity type. Since layer 240 has a lower bandgap, it alsohas a higher electrical conductivity than either of adjacent layers 224and 230. In general, while layers 224 and 230 can each have a bandgap onthe order of 1.9 eV, the bandgap of layer 240 is preferably on the orderof less than 1.7 eV.

In accordance with a first variation of the first embodiment, interfacelayer 240 is provided as a p⁺ -type hydrogenated amorphous silicon layersubstantially free of carbon, nitrogen, fluorine or similar bandgapwidening elements. Layer 240 so provided preferably has a thickness inthe range of 10 Å to 200 Å and preferably approximately 20 Å, and ispreferably doped with an acceptor dopant, such as boron, to aconcentration of approximately 1%. As a result, layer 240 provided as ap⁺ -type layer forms a highly conductive, ohmic contact with p-typelayer 230 of cell 212 and a tunnel junction with n-type layer 224 ofcell 210.

In accordance with a second variation of the first embodiment, interfacelayer 240 is provided as a microcrystalline layer of n⁺ -typehydrogenated silicon that is substantially free of carbon, nitrogen,fluorine or other bandgap widening elements. As used herein,microcrystalline describes a material whose crystalline order is in therange of substantially 25 Å to 500 Å. The presence of microcrystallinitycan be determined by one or more of electron diffraction, x-raydiffraction, or Raman scattering spectroscopy. Since dopedmicrocrystalline silicon is an indirect bandgap material having arelatively low bandgap, e.g., approximately 1.1 eV, it provides the dualadvantages of high conductivity and high optical transmissivity.

Layer 240 provided as a microcrystalline n⁺ -type layer preferably has athickness in the range of 15 Å to 50 Å and preferably approximately 50Å, and is preferably doped with a donor dopant, such as phosphorus, to aconcentration of approximately 1%. As a result, layer 240 provided as ann⁺ -type microcrystalline layer forms a highly conductive, ohmic contactwith n-type layer 224 of cell 210 and a tunnel junction with p-typelayer 230 of cell 212. Layer 240 provided as a microcrystalline layerhas a relatively low optical bandgap but retains a high opticaltransmissivity because it is an indirect optical bandgap material.

While the first embodiment of the present invention can be successfullyconstructed and practiced with n-type layer 224 of cell 210 providedwith a uniform carbon concentration throughout, it is preferred hereinthat the carbon concentration of layer 224 be graded from a maximumvalue at the interface with adjacent intrinsic layer 222 to a minimumvalue, preferably a zero concentration, at the interface with layer 240.Such grading can be achieved by either a stepped or continuous gradingof the carbon concentration. As discussed above, carbon is included inn-type layer 224, across all or part of the thickness of layer, in orderto increase the optical bandgap of that layer. However,disadvantageously, the presence of carbon decreases the conductivity ofeach of layers 224 and 230. For this reason, the carbon concentration oflayer 224 is preferably graded so that the bandgap and hence theconductivity of layer 224 has a maximum value at the interface withlayer 240 and can better form a conductive contact therewith. Further,with respect to the first variation of the first embodiment, theenhanced conductive contact between a graded n-type layer 224 and layer240 provided as a p⁺ -type layer results in improved performance of thetunnel junction formed therebetween.

It is noted that p-type layer 230 of cell 212 can also be provided witha carbon concentration that is graded, by steps or continuously, from amaximum value at its interface with intrinsic layer 232 to a minimumvalue, e.g., zero concentration, at the interface with layer 240. Suchgrading of layer 230 improves its conductive contact with layer 240.However, the present inventors have observed that the open circuitvoltage of a cell decreases when the p-layer is graded, while the cellopen circuit voltage is not particularly sensitive to grading of then-layer.

In accordance with a third variation of the first embodiment, n-typelayer 224 has a graded carbon concentration. Further, layer 240 consistsof a first sublayer of n⁺ -type a-Si:H, adjacent to layer 224, and asecond sublayer of microcrystalline n⁺ -type layer. The second sublayeris disposed between the first sublayer and layer 230 of cell 212 and isotherwise as described above with respect to the second variation of thefirst embodiment. In accordance with this third variation, the a-Si:H n⁺-type sublayer is substantially free of bandgap widening elements, has athickness in the range of 10 Å to 100 Å and preferably 15 Å, and ispreferably doped with phosphorus to a concentration of approximately 1%.

In accordance with the first embodiment, the tunnel junction formedbetween interface layer 240 and one of layers 224 and 230 functions witha substantially resistive characteristic so that the IV characteristicof device 200 is represented by curve 150 in FIG. 1. Advantageously, ascan be seen, curve 150 does not include the inflection found in curve100. As a result, device 200 constructed to include a tunnel junction inaccordance with the first embodiment of the invention, enjoys animproved fill factor and power generation capability.

Also in accordance with the first embodiment, in contrast to the priorart metallic layer described in the above referenced '641 patent,interface layer 240 is comprised of a silicon compound havingsubstantially the same index of refraction as adjacent layers 224 and230. As a result, device 200 does not experience undesirablereflectivity at the interfaces with layer 240.

Methods for constructing each of the first and second variations of thefirst embodiment are described next. In this regard, only the formationof layers 224, 230, and 240 are described in detail. A suitable methodfor constructing the remainder of device 200 is disclosed in theabove-incorporated U.S. Pat. No. 4,272,641.

One possible method for constructing device 200 in accordance with thefirst variation of the first embodiment is described first. Allsemiconductor layers of device 200 are formed by a glow dischargedeposition process. The described method includes a method for providingeach of n-type layer 224 and p-type layer 230 with a graded carbonconcentration.

First, layer 224 is formed by depositing successively narrower opticalbandgap a-SiC alloys by gradually removing carbon during the depositionprocess. In particular, using silane, 4.5% phosphine in silane andmethane, deposition of the wide bandgap portion of n-layer 224, adjacentintrinsic layer 222, begins with a mixture of 125 sccm PH₃ -SiH₄ +25sccm SiH₄ +25 sccm methane. Layer 224 is deposited by either dc or rfglow discharge using a power of 0.4 watts/in² yielding a deposition rateof about 4 Å/second. Two or more layers, or a continuous grading out ofcarbon, are deposited in succession with the carbon being graduallyremoved, producing a total thickness of layer 224 of nominally 100 Å.For example, the stepped grading of carbon can be accomplished overthree layers. The final gas mixture is 125 sccm PH₃ -SiH₄ +75 sccm SiH₄.Layer 240 provided as a p⁺ -type layer is then deposited by glowdischarge to a 20 Å thickness from a mixture of 100 sccm 0.5%diborane-SiH₄ +100 sccm SiH₄. Last, p-type a-SiC:H layer 230 isdeposited by glow discharge to a thickness of approximately 100 Å.Assuming layer 230 is deposited with a graded optical bandgap, the layeris initially deposited from a mixture of 30 sccm 0.5% diborane-SiH₄ +90sccm SiH₄. Two or more layers, or a continuous grading of carbon, aredeposited in succession so that the final mixture from which the layeris deposited includes 80 sccm methane. If the bandgap of layer 230 isnot graded, the mixture continuously includes 80 sccm methane. Suitableflushes are provided between depositions to avoid cross contamination.However, the entire fabrication process is carried out with a singleglow discharge deposition system.

One possible method for constructing device 200 in accordance with thesecond variation of the first embodiment is described next. Again, alllayers of device 200 are formed by a glow discharge deposition process.In accordance with the method described next, both layers 224 and 230are provided with a uniform carbon concentration. First, n-type a-SiC:Hlayer 224 is deposited by dc or rf discharge to a thickness of about 100Å using 125 sccm 4.5%PH₃ -SiH₄ mixture+25 sccm CH₄ +75 sccm SiH₄ usingapproximately 0.4 watts/in² at about 0.5 torr.

Layer 240 as a microcrystalline n-type layer is deposited next. Theformation of a microcrystalline doped layer has in the past beendifficult to achieve for the extremely thin (10-100 Å) layers requiredto maintain high optical transmission. This limitation has been overcomeby using a high intensity hydrogen glow discharge process to crystallizean amorphous layer which then acts as the "seed" nuclei for the furthergrowth of the crystalline phase. In particular, a 20 Å thick non-carboncontaining n-type layer of hydrogenated p-type silicon is depositedusing gas flow rates of 125 sccm of 4.5% PH₃ -SiH₄ +100 sccm SiH₄. A 4second glow discharge deposition at 0.5 torr and 0.4 watts/in² is usedto form the layer. Next, the amorphous deposited layer is recrystallizedby using a hydrogen plasma created at 1.5 torr (320 sccm flow rate) for5 minutes at an rf or dc power level of approximately 2 watts/in² Themicrocrystalline n-layer is deposited at 1.5 torr using a 2 watt/in²glow discharge with a flow rate of 5 sccm of 4.5% PH₃ in SiH₄ diluted ina 320 sccm flow of pure hydrogen. The deposition is conducted for 50seconds yielding an approximate thickness of 50 Å. Last, p-type a-SiClayer 230 of cell 212 is deposited for 17 seconds at 0.5 torr from a gasmixture of 25 sccm 0.5% B₂ H₆ in SiH₄ +80 sccm methane+100 sccm SiH₄ atan rf or dc power level of approximately 0.4 watt/in².

With respect to the construction of the third variation of the firstembodiment, the carbon concentration of layer 224 is graded out in themanner described above. Then, the n⁺ -type a-Si:H sublayer is depositedby a glow discharge to a thickness of approximately 100 Å from a mixtureof 125 sccm PH₃ -SiH₄ +75 sccm SiH₄. Next, the n⁺ -type microcrystallinelayer and p⁺ -type layer 230 are deposited as described above for thesecond variation of the first embodiment.

FIG. 3 illustrates a multijunction photovoltaic device 300 constructedin accordance with a second embodiment of the invention. Device 300includes substrate 202, front conductive layer 208 and photovoltaiccells 210 and 212 as described above for device 200. Device 300 alsoincludes an interface layer 302 comprised of layers 304 and 306sandwiched between n-type layer 224 of device 210 and p-type layer 230of device 212 such that layer 304 is in contact with layer 224 and layer306 is in contact with layer 230. In accordance with several variationsof the second embodiment, layers 304 and 306 are respectively providedas n-type and p-type hydrogenated silicon layers each having arelatively low bandgap. In particular, each of layers 304 and 306 has alower bandgap, and therefore a higher conductivity, than layers 224 and230. As a result, in accordance with these variations, n-layer 304 formsa highly conductive, ohmic contact with n-type layer 224, p-type layer306 forms a highly conductive, ohmic contact with p-type layer 230 and atunnel junction is formed between layers 304 and 306. The bandgap ofeach of layers 304 and 306 is on the order described above for layer 240of the first embodiment.

In accordance with a first variation of the second embodiment, layer 304is provided as a microcrystalline layer of n⁺ -type hydrogenated siliconthat is substantially free of carbon, nitrogen, fluorine or otherbandgap widening elements. Layer 304 has a thickness in the range of 10Å to 100 Å and preferably approximately 20 Å, and a dopant concentrationof approximately 1%. Layer 306 is provided as a layer of p⁺ -typehydrogenated amorphous silicon that is substantially free of the abovenoted bandgap widening elements. Layer 306 has a thickness in the rangeof 5 Å to 100 Å and preferably approximately 15 Å, and a dopantconcentration of approximately 1%.

In accordance with a second variation of the second embodiment, layer304 is provided as a layer of n⁺ -type hydrogenated amorphous siliconthat is substantially free of the above-noted bandgap widening elements.Layer 304 so provided has a thickness in the range of 10 Å to 100 Å andpreferably approximately 15 Å, and a dopant concentration ofapproximately 1%. Layer 306 of this variation is the same as describedabove for the first variation.

In accordance with a third variation of the second embodiment, layer 304is provided as a microcrystalline layer of n⁺ -type hydrogenated siliconas described above for the first variation. Also in accordance with thethird variation, layer 306 is provided as a microcrystalline layer of p⁺-type hydrogenated silicon that is substantially free of the above-notedbandgap widening elements. In accordance with this variation, layer 306has a thickness in the range of 10 Å to 100 Å and preferablyapproximately 15 Å, and a dopant concentration of approximately 1%.

In accordance with a fourth variation of the second embodiment, layer306 is provided as described for the third variation and layer 304 isprovided as described for the second variation.

As in the case of the first embodiment, each variation of the secondembodiment can be successfully constructed and practiced with n-typelayer 224 and p-type layer 230 each provided with a uniform carbonconcentration throughout. However, it is preferred that the carbonconcentration of layer 224 be graded as described above with respect tothe first embodiment. The carbon concentration of p-layer 230 can alsobe graded as previously described.

In accordance with the second embodiment of the invention, thetunnel-junction formed between layers 304 and 306 functions with asubstantially resistive characteristic so that the IV characteristic ofdevice 300, like device 200, is also represented by curve 150 in FIG. 1.As a result, device 300 constructed in accordance with the secondembodiment of the invention enjoys an improved fill factor and powergeneration capability. Also, as in the case of the first embodiment,layers 304 and 306 are each comprised of a silicon compound havingsubstantially the same index of refraction as layers 224 and 230. As aresult, device 300 does not experience undesirable reflectivity at theinterfaces with layer 302.

Methods for constructing device 300 are described next. As in the caseof the first embodiment, all layers of device 300 are formed by a glowdischarge deposition process. With the exception of the microcrystallinep⁺ -type layer included in the third and fourth variations of the secondembodiment, the methods described above for constructing layer 240 ofthe first embodiment provided as p⁺ -type a-Si:H, n⁺ -typemicrocrystalline silicon, or n⁺ -type a-Si:H, are fully applicable tolayers 304 and 306. The p⁺ -type microcrystalline layer 306 of the thirdand fourth variations can be formed in a manner analogous to thatdescribed above for forming the n⁺ -type microcrystalline layer. Forexample, layer 306 can be deposited from a mixture of 0.5% diborane insilane diluted with 320 sccm hydrogen, by glow discharge with a power of2 watts/in².

While devices 200 and 300 constructed in accordance with embodiments ofthe invention have been described as including carbon in theirrespective n-type and p-type layers as a bandgap widening element, theinvention is not so limited. The invention can be practiced with equaleffectiveness using a bandgap widening element other than carbon, e.g.,fluorine, nitrogen, or oxygen.

The following example illustrates the advantages realized fromconstructing a multijunction photovoltaic device to include an interfacelayer that forms a tunnel junction with an adjacent cell layer. In theexample, two multijunction devices, designated device #1 and device #2,were constructed. Both devices #1 and #2 included two p-i-n typephotovoltaic cells having characteristics as described above for theillustrated embodiments. Device #1 did not include an interface layer.Device #2 included an interface layer such as described above for thefirst variation of the first embodiment of the invention. Thus, theinterface layer was comprised of a boron doped p⁺ -type hydrogenatedamorphous silicon layer substantially free of carbon, nitrogen, fluorineor other bandgap widening elements. In particular, in device #2, theinterface layer had a thickness of approximately 20 Å and a boronconcentration of approximately 0.3%. Thus, devices #1 and #2 wereidentical except for the inclusion of the interface layer in device #2.

Table 1 lists performance characteristics of devices #1 and #2 measuredunder identical irradiation conditions.

                  TABLE 1                                                         ______________________________________                                                   Device #1   Device #2                                              ______________________________________                                        Voc           2.141 volts   2.225 volts                                       Jsc          -6.4 ma/cm.sup.2                                                                            -6.5 ma/cm.sup.2                                   Fill Factor   58.1%         63.1%                                             Efficiency    7.9%          9.2%                                              Rs           129.16 ohm-cm.sup.2                                                                          54.89 ohm-cm.sup.2                                ______________________________________                                    

In Table 1, Voc and Jsc are respectively the open circuit voltage andshort circuit current density of the device. As seen in Table 1, theinclusion of the interface layer in device #2 results in an 8.6%increase in fill factor which is reflected by the corresponding increasein device efficiency. As also seen in Table 1, the inclusion of theinterface layer in device #2 results in a substantial reduction in theseries resistance of the multi junction device.

While devices 200 and 300 each include only two photovoltaic cells, theinvention can be practiced with multi junction photovoltaic devices thatinclude more than two cells. In the subsequent embodiments, an interfacelayer constructed in accordance with either the first or secondembodiment of the invention is interposed between one or more ofadjacent photovoltaic cells in a multijunction device.

FIG. 4 illustrates a multijunction photovoltaic device 400 constructedin accordance with a third embodiment of the invention. Device 400includes substrate 202, front conductive layer 208, photovoltaic cell210, interface layer 240, and back contact layer 236 as described abovefor devices 200 and 300. Additionally, device 400 includes a secondphotovoltaic cell 410, and third photovoltaic cell 412, arranged in astacked configuration and sandwiched between interface layer 240 andback contact layer 236.

Photovoltaic cell 410 has substantially the same basic structure as thatof photovoltaic cell 210, in that it is comprised of p-type a-SiC:Hlayer 230, an intrinsic hydrogenated amorphous silicon layer 420, and ann-type a-SiC:H layer 234. Photovoltaic cell 412 also has substantiallythe same basic structure as that of photovoltaic cell 210, in that it iscomprised of p-type a-SiC:H layer 422, an intrinsic hydrogenatedamorphous silicon layer 424, and an n-type a-SiC:H layer 426. The p- andn-type layers, of cells 210, 410, and 412, include carbon concentrationsof approximately 20% and 15%, respectively, at least adjacent to theintrinsic layer in each cell. It is noted that while both the p- andn-type layers of cells 210, 410, and 412 include carbon, a multijunctiondevice can be constructed in accordance with the invention withoutincluding a bandgap widening element in all doped layers.

While the intrinsic layer 232 of the second photovoltaic cell 212 in thefirst and second embodiments was described as being substantially freeof bandgap widening elements, the intrinsic layer 420 of the secondphotovoltaic cell 410 in the third embodiment is formed of an a-SiGealloy in order to alter the bandgap to be lower than that of intrinsiclayer 222 of the first photovoltaic cell 210. Similarly, intrinsic layer424 of the third photovoltaic cell 412 is formed of an a-SiGe alloyhaving a higher ratio of Ge to silicone than intrinsic layer 420 of thesecond photovoltaic cell 410 in order to alter the bandgap such that itis lower than that of intrinsic layer 420. For example, if intrinsiclayer 222 has a bandgap of approximately 1.73 eV, then intrinsic layer420 would preferably have a bandgap between 1.45 and 1.65 eV, andintrinsic layer 424 would preferably have a bandgap between 1.20 and1.45 eV. If intrinsic layer 222 includes a bandgap widening element andhas a bandgap of approximately 1.99 eV, then intrinsic layer 420 wouldpreferably have a bandgap between 1.45 and 1.75 eV, and intrinsic layer424 would preferably have a bandgap between 1.20 and 1.45 eV.

By narrowing the bandgap of the intrinsic layer of each subsequentphotovoltaic cell, the cells may be tuned to absorb light of differentwavelengths. A drawback to this approach is that, as the bandgap isnarrowed, the voltage produced across the cell is lowered. However, suchdrops in voltage may be compensated for by increases in the photocurrentthat are generated by individually tuning each cell to absorb differentwave bands of the incident light.

By adding Ge into intrinsic layers 420 and 424 to form a-SiGe alloyshaving successively lower bandgaps, the efficiency of intrinsic layers420 and 424 can be increased. Hence, intrinsic layers 420 and 424 may bemade thinner without reducing the efficiency of the device. Bydecreasing the thickness of intrinsic layers 420 and 424, the stabilityof these layers is increased. Further, since the intrinsic layers of thesecond and third cells were typically the least stable layers in amultijunction photovoltaic device due to their required thicknesses, theoverall stability of the entire photovoltaic device can be improved byintroducing Ge into the intrinsic layers of the second and third cells.

Furthermore, by definition, an increase in the stability of the devicemeans that the efficiency of the device does not drop as dramaticallyover years of use due to photodegradability and the like. Thus, althoughthe initial efficiency of a device having an a-SiGe intrinsic layer maybe lowered as a result of making the intrinsic layer thinner, theefficiency of the device after a number of years have passed willactually be greater than a device having a thicker a-Si intrinsic layer.

There are a number of additional benefits of making the intrinsic layersthinner such as: providing a more compact photovoltaic device; usingless materials to construct the device; and lowered cost of manufacture.Also, when a photovoltaic device is not very stable, it is necessary tomake continued adjustments in the connected circuitry to ensure that asteady level of power is supplied to a consumer. Such adjustments aretime consuming and costly. Therefore, by providing a more stablephotovoltaic device, time and money can be saved.

In Table 2 below, constructional and performance data of numerouscalculated examples of multijunction photovoltaic devices are provided.In constructing the data provided in Table 2, the following assumptionswere made: (1) open-circuit voltage (V_(oc))=0.5 ΣE_(g) (bandgap) andinitial fill factor (FF)=0.7, for case numbers 1-6 and 12-26; (2) allmaterials degrade at the same rate, namely that of a-Si; (3) there is nodegradation in V_(oc) ; and (4) degradation in efficiency is linear withi-layer thickness, 1%/100 Å in 1000 hours. Using the above assumptionsand varying the values of the thicknesses and bandgaps of threeintrinsic layers, the estimated initial values of short-circuit current(Jsc) and efficiency (Eff) can be calculated. To calculate the finalefficiency (Eff), the efficiency of the device is estimated based upon1000 hours of continuous exposure to white light having an intensity of100 mW/cm² which is approximately the intensity of sunlight. Thisexposure is a good approximation of the effect of deploying the celloutdoors for one year. The rate of photodegradation is stronglydependent on the intensity of the incident radiation and can beapproximated from the equation, (time)(intensity)¹.8 =constant.

                                      TABLE 2                                     __________________________________________________________________________    I.sub.1    I.sub.2                                                                             I.sub.3                                                                             Initial      Final        Final                        Case #                                                                            Eg T   Eg T  Eg T  Voc                                                                              FF  Jsc                                                                              Eff                                                                              Voc                                                                              FF  Jsc                                                                              Eff                                                                              Initial                      __________________________________________________________________________     1  1.99                                                                             2100                                                                              1.70                                                                             4000                                                                             1.40                                                                              900                                                                             2.55                                                                             0.7 6.29                                                                             11.23                                                                            2.55                                                                             0.598                                                                             6.02                                                                             9.19                                                                             0.818                         2  1.99                                                                             3000                                                                              1.55                                                                             3000                                                                             1.40                                                                             3300                                                                             2.46                                                                             0.7 7.39                                                                             12.74                                                                            2.46                                                                             0.573                                                                             6.96                                                                             9.81                                                                             0.770                         3  1.99                                                                             3600                                                                              1.55                                                                             2200                                                                             1.40                                                                             2000                                                                             2.46                                                                             0.7 7.02                                                                             12.10                                                                            2.46                                                                             0.603                                                                             6.74                                                                             9.99                                                                             0.826                         4  1.99                                                                             2200                                                                              1.70                                                                             4000                                                                             1.20                                                                              250                                                                             2.45                                                                             0.7 6.4                                                                              10.99                                                                            2.45                                                                             0.600                                                                             6.15                                                                             9.04                                                                             0.823                         5  1.99                                                                             3000                                                                              1.55                                                                             2100                                                                             1.20                                                                              400                                                                             2.36                                                                             0.7 7.07                                                                             11.69                                                                            2.36                                                                             0.615                                                                             6.82                                                                             9.89                                                                             0.846                         6  1.99                                                                             3000                                                                              1.40                                                                              800                                                                             1.20                                                                              450                                                                             2.30                                                                             0.7 7.08                                                                             11.44                                                                            2.30                                                                             0.627                                                                             6.86                                                                             9.90                                                                             0.865                         7  1.73                                                                             600 1.73                                                                             6000                                                                             1.40                                                                             1200                                                                             2.18                                                                             0.694                                                                             6.37                                                                             10.04                                                                            2.18                                                                             0.57                                                                              5.97                                                                             7.42                                                                             0.739                         8  1.73                                                                             600 1.73                                                                             4000                                                                             1.40                                                                             1200                                                                             2.18                                                                             0.741                                                                             5.54                                                                             9.31                                                                             2.18                                                                             0.64                                                                              5.39                                                                             7.52                                                                             0.808                         9  1.73                                                                             600 1.68                                                                             4000                                                                             1.40                                                                             1200                                                                             2.15                                                                             0.685                                                                             6.44                                                                             9.89                                                                             2.15                                                                             0.60                                                                              6.20                                                                             8.00                                                                             0.809                        10  1.73                                                                             600 1.68                                                                             3000                                                                             1.40                                                                             1200                                                                             2.15                                                                             0.724                                                                             5.82                                                                             9.44                                                                             2.15                                                                             0.63                                                                              5.67                                                                             7.68                                                                             0.813                        11  1.73                                                                             600 1.63                                                                             2500                                                                             1.40                                                                             1200                                                                             2.12                                                                             0.69                                                                              6.33                                                                             9.64                                                                             2.12                                                                             0.62                                                                              6.14                                                                             8.07                                                                             0.837                        12  1.73                                                                             950 1.55                                                                             4600                                                                             1.40                                                                             7500                                                                             2.33                                                                             0.7 7.79                                                                             12.71                                                                            2.33                                                                             0.522                                                                             7.07                                                                             8.6                                                                              0.677                        13  1.73                                                                             825 1.55                                                                             3000                                                                             1.40                                                                             3500                                                                             2.33                                                                             0.7 7.4                                                                              12.08                                                                            2.33                                                                             0.601                                                                             7.09                                                                             1.92                                                                             0.821                        14  1.73                                                                             780 1.55                                                                             2750                                                                             1.40                                                                             3000                                                                             2.33                                                                             0.7 7.29                                                                             11.9                                                                             2.33                                                                             0.611                                                                             7.04                                                                             10.02                                                                            0.842                        15  1.73                                                                             750 1.55                                                                             2500                                                                             1.40                                                                             2500                                                                             2.33                                                                             0.7 7.17                                                                             11.69                                                                            2.33                                                                             0.622                                                                             6.95                                                                             10.07                                                                            0.861                        16  1.73                                                                             720 1.55                                                                             2250                                                                             1.40                                                                             2200                                                                             2.33                                                                             0.7 7.04                                                                             11.5                                                                             2.33                                                                             0.633                                                                             6.86                                                                             10.13                                                                            0.881                        17  1.73                                                                             675 1.55                                                                             2000                                                                             1.40                                                                             1800                                                                             2.33                                                                             0.7 6.87                                                                             11.22                                                                            2.33                                                                             0.64                                                                              6.72                                                                             10.03                                                                            0.894                        18  1.73                                                                             1200                                                                              1.55                                                                             7500                                                                             1.20                                                                             3300                                                                             2.23                                                                             0.7 8.51                                                                             13.28                                                                            2.23                                                                             0.521                                                                             7.7                                                                              8.94                                                                             0.674                        19  1.73                                                                             850 1.55                                                                             3000                                                                             1.20                                                                              700                                                                             2.23                                                                             0.7 7.49                                                                             11.69                                                                            2.23                                                                             0.633                                                                             7.3                                                                              10.3                                                                             0.881                        20  1.73                                                                             820 1.55                                                                             2750                                                                             1.20                                                                              630                                                                             2.23                                                                             0.7 7.41                                                                             11.56                                                                            2.23                                                                             0.638                                                                             7.23                                                                             10.3                                                                             0.891                        21  1.73                                                                             780 1.55                                                                             2500                                                                             1.20                                                                              580                                                                             2.23                                                                             0.7 7.3                                                                              11.43                                                                            2.23                                                                             0.643                                                                             7.14                                                                             10.23                                                                            0.895                        22  1.73                                                                             750 1.55                                                                             2250                                                                             1.20                                                                              500                                                                             2.23                                                                             0.7 7.17                                                                             11.19                                                                            2.23                                                                             0.645                                                                             7.03                                                                             10.18                                                                            0.910                        23  1.73                                                                             700 1.55                                                                             2000                                                                             1.20                                                                              450                                                                             2.23                                                                             0.7 6.95                                                                             10.92                                                                            2.23                                                                             0.657                                                                             6.83                                                                             10.01                                                                            0.917                        24  1.73                                                                             1350                                                                              1.45                                                                             3000                                                                             1.20                                                                             1400                                                                             2.20                                                                             0.7 8.63                                                                             13.30                                                                            2.20                                                                             0.620                                                                             8.36                                                                             11.39                                                                            0.856                        25  1.73                                                                             1150                                                                              1.45                                                                             2700                                                                             1.20                                                                             1300                                                                             2.20                                                                             0.7 8.42                                                                             12.96                                                                            2.20                                                                             0.629                                                                             8.19                                                                             11.33                                                                            0.874                        26  1.73                                                                             700 1.45                                                                             1000                                                                             1.20                                                                              450                                                                             2.20                                                                             0.7 6.96                                                                             10.79                                                                            2.20                                                                             0.674                                                                             6.89                                                                             10.22                                                                            0.947                        __________________________________________________________________________

In Table 2, the value "Final/Initial" represents the stability of thedevice estimated after one year of exposure, and is a ratio equal to thefinal efficiency divided by the initial efficiency. Although it isdesirable to design photovoltaic devices to have the maximum long-termefficiency, it may be more desirable to design the photovoltaic deviceto have the maximum final/initial ratio to ensure a high level ofefficiency for the long term use of the device.

Many of the aforementioned advantages of the utilizing SiGe alloys inthe intrinsic layers are evidenced from the data provided in Table 2.For example, although providing a thicker middle intrinsic layer resultsin a higher initial efficiency, it is readily apparent by comparingcases 18 and 19, that a device constructed with a thinner middleintrinsic layer will exhibit a higher final efficiency due to the higherlevel of stability that is achieved. Further, by comparing cases 8 and9, it will be apparent that narrowing the bandgap of the middleintrinsic layer results in a higher initial efficiency and a higherfinal efficiency.

As apparent from Table 2, it can generally be expected that thelong-term efficiency will be at least 80% of the initial efficiency whenthe thickness of the middle intrinsic layer is less than 3000 Å.

Device 400 may optionally include a second interface layer 411 ofrelatively low bandgap, high conductivity semiconductor materialinterposed between the n-type a-SiC:H layer 234 of photovoltaic cell 410and the p-type a-SiC:H layer 422 of photovoltaic cell 412. Like firstinterface layer 240, second interface layer 411 has a lower bandgap thaneither of its adjacent layers 234 or 422 and can be provided as a p- orn-type semiconductor material that forms an ohmic contact with theadjacent one of layers 234 and 422 of the same conductivity type andthat forms a tunnel junction with the one of layers 234 and 422 of theopposite conductivity type. Since layer 411 has a lower bandgap, it alsohas a higher electrical conductivity than either of adjacent layers 234and 422. In general, while layers 234 and 422 can each have a bandgap onthe order of 1.9 eV, the bandgap of layer 411 is preferably on the orderof less than 1.7 eV.

Although the third embodiment has been described as including aninterface layer 240 sandwiched between the first and second photovoltaiccells 210 and 410, it is considered within the scope of the presentinvention to form device 400 including second interface layer 411 formedbetween the second and third photovoltaic layers, but not includinginterface layer 240. However, given the benefits in providing interfacelayers between each junction of a multijunction photovoltaic device, itis preferable to include both interface layers 240 and 411.

Many variations of the third embodiment are possible in view of thenumber of possible combinations of the variations discussed above withrespect to the first embodiment. For example, a multijunction device maybe constructed in accordance with the third embodiment to include afirst interface layer between first and second photovoltaic cells and asecond interface layer formed between second and third photovoltaiccells, wherein the first interface layer is constructed in accordancewith one variation of the first embodiment (e.g., interface layer 240 isprovided as a p⁺ -type hydrogenated amorphous silicon layer), and thesecond interface layer is constructed in accordance with anothervariation of the first embodiment (e.g., interface layer 411 is providedas a microcrystalline layer of n⁺ -type hydrogenated silicon). Althoughfurther examples of the possible variations of the third embodiment arenot provided herein, such variations are to be considered within thescope of the third embodiment.

One possible method for constructing the third embodiment is describednext. In this regard, only the formation of layers 420 and 424 aredescribed in detail. A suitable method for constructing the remainder ofdevice 400 is disclosed above with respect to the first embodiment.Layers 411, 422, and 426 of device 400, which are not included in thefirst embodiment, may be constructed in a similar manner by whichcorresponding layers 240, 230, and 234 are respectively constructed asdescribed above with respect to the first embodiment. Intrinsic layer222 of the first photovoltaic cell typically is formed to have athickness between 600 and 700 Å.

Similar to the construction of the first embodiment, all semiconductorlayers of device 400 are formed by a glow discharge deposition process.Intrinsic layer 420 is deposited by dc or rf discharge to a thickness ofabout 1,500 to 2,500 Å using 25 sccm 20%GeH₄ :H mixture+75 sccm SiH₄using approximately 0.25 watts/in² at about 1.5 torr. Intrinsic layer424 is deposited by dc or rf discharge to a thickness of about 1,500 to2,500 Å using 50 sccm 20%GeH₄ :H mixture+50 sccm SiH₄ usingapproximately 0.25 watts/in² at about 1.5 torr.

FIG. 5 illustrates a multijunction photovoltaic device 500 constructedin accordance with a fourth embodiment of the invention. Device 500includes substrate 202, front conductive layer 208, first photovoltaiccell 210, second photovoltaic cell 410, second interface layer 411,third photovoltaic cell 412, and back contact layer 236 as describedabove for device 400. The only difference between device 500 and device400 of the third embodiment is that interface layer 240 has beenreplaced with interface layer 302 which is described above with respectto the second embodiment.

As described above in the second embodiment, interface layer 302includes layer 304 and 306. Again, layers 304 and 306 may include any ofthe possible variations and combinations provided for in the secondembodiment.

Many variations of the fourth embodiment are possible in view of thenumber of possible combinations of the variations discussed above withrespect to interface layer 240, which also apply to interface layer 411,in the first embodiment, and the variations discussed above with respectto interface layer 302 in the second embodiment. For example, amultijunction device may be constructed in accordance with the fourthembodiment to include a first interface layer between first and secondphotovoltaic cells and a second interface layer formed between secondand third photovoltaic cells, wherein the first interface layer isconstructed in accordance with one of the variations of the secondembodiment (e.g., layer 304 is provided as a microcrystalline layer ofn⁺ -type hydrogenated silicon, and layer 306 is provided as a layer ofp⁺ -type hydrogenated amorphous silicon), and the second interface layeris constructed in accordance with one of the variations of the firstembodiment (e.g., interface layer 411 is provided as a microcrystallinelayer of n⁺ -type hydrogenated silicon). Although further examples ofthe possible variations of the fourth embodiment are not providedherein, such variations are to be considered within the scope of thefourth embodiment.

FIG. 6 illustrates a multijunction photovoltaic device 600 constructedin accordance with a fifth embodiment of the invention. Device 600includes substrate 202, front conductive layer 208, first photovoltaiccell 210, the first interface layer 240, second photovoltaic cell 410,third photovoltaic cell 412, and back contact layer 236 as describedabove for device 400. The only difference between device 600 and device400 of the third embodiment is that interface layer 411 has beenreplaced with interface layer 602 which corresponds to interface layer302 described above with respect to the second embodiment.

Like interface layer 302 of the second embodiment, interface layer 602includes layers 604 and 606. Layers 604 and 606 may include any of thepossible variations and combinations provided for in the secondembodiment with respect to layers 304 and 306.

Many variations of the fifth embodiment are possible in view of thenumber of possible combinations of the variations discussed above withrespect to interface layer 240 in the first embodiment, and thevariations discussed above with respect to the second interface layer602 which correspond to the variations of interface layer 302 in thesecond embodiment. For example, a multijunction device may beconstructed in accordance with the fifth embodiment to include a firstinterface layer between first and second photovoltaic cells and a secondinterface layer formed between second and third photovoltaic cells,wherein the first interface layer is constructed in accordance with oneof the variations of the first embodiment (e.g., interface layer 240 isprovided as a p⁺ -type hydrogenated amorphous silicon layer), and thesecond interface layer is constructed in accordance with one of thevariations of the second embodiment (e.g., layer 604 is provided as amicrocrystalline layer of n⁺ -type hydrogenated silicon, and layer 606is provided as a layer of p⁺ -type hydrogenated amorphous silicon).Although further examples of the possible variations of the fifthembodiment are not provided herein, such variations are to be consideredwithin the scope of the fifth embodiment.

FIG. 7 illustrates a multijunction photovoltaic device 700 constructedin accordance with a sixth embodiment of the invention. Device 700includes substrate 202, front conductive layer 208, first photovoltaiccell 210, second photovoltaic cell 410, second interface layer 602,third photovoltaic cell 412, and back contact layer 236 as describedabove for device 600. The only difference between device 700 and device600 of the fifth embodiment is that first interface layer 240 has beenreplaced with interface layer 302 which is described above with respectto the second embodiment.

Many variations of the sixth embodiment are possible in view of thenumber of possible combinations of the variations discussed above withrespect to interface layer 302 in the second embodiment, and thevariations discussed above with respect to the second interface layer602 which also correspond to the variations of interface layer 302 inthe second embodiment. For example, a multijunction device may beconstructed in accordance with the sixth embodiment to include a firstinterface layer between first and second photovoltaic cells and a secondinterface layer formed between second and third photovoltaic cells,wherein the first interface layer is constructed in accordance with oneof the variations of the second embodiment (e.g., layer 304 is providedas a layer of n⁺ -type hydrogenated amorphous silicon, and layer 306 isprovided as a layer of p⁺ -type hydrogenated amorphous silicon), and thesecond interface layer is constructed in accordance with anothervariation of the second (e.g., layer 604 is provided as amicrocrystalline layer of n⁺ -type hydrogenated silicon, and layer 606is provided as a layer of p⁺ -type hydrogenated amorphous silicon).Although further examples of the possible variations of the sixthembodiment are not provided herein, such variations are to be consideredwithin the scope of the sixth embodiment.

The foregoing description of the preferred embodiments and examples ofthe invention have been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications andvariations are possible in light of the above teachings or may beacquired from practice of the invention. The embodiments were chosen anddescribed in order to explain the principles of the invention and itspractical application to enable one skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

What is claimed is:
 1. A multijunction photovoltaic device, comprising:atransparent substrate; a transparent electrode formed on saidtransparent substrate; a first photovoltaic cell formed on saidtransparent electrode and comprising;a p-type layer formed adjacent saidtransparent electrode, an intrinsic layer of an amorphous semiconductorcompound including silicon formed on said p-type layer, and an n-typelayer formed on said intrinsic layer; an interface layer of asemiconductor compound including silicon formed on said n-type layer ofsaid first photovoltaic cell, said interface layer having a bandgap lessthan the bandgap of said n-type layer of said first photovoltaic cell; asecond photovoltaic cell formed on said interface layer and comprising,ap-type layer formed on said interface layer and having a bandgap greaterthan the bandgap of said interface layer, an intrinsic layer formed onsaid p-type layer, having a thickness less than 3000 Å, and being madeof an amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said first photovoltaic cell, and an n-type layer formed onsaid intrinsic layer; a third photovoltaic cell formed on said secondphotovoltaic cell and comprising,a p-type layer, an intrinsic layer ofan amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said second photovoltaic cell, formed on said p-type layer, andan n-type layer formed on said intrinsic layer; and a reflectiveelectrode layer formed on said n-type layer of said third photovoltaiccell.
 2. The multijunction device of claim 1, wherein the intrinsiclayer of said first photovoltaic cell has a thickness between 600 and700 Å.
 3. The multijunction device of claim 1, wherein the intrinsiclayer of said third photovoltaic cell has a thickness between 1,500 and2,500 Å.
 4. The multijunction device of claim 1, wherein the bandgap ofsaid intrinsic layer of said first photovoltaic cell is between 1.7 and1.99 eV.
 5. The multijunction device of claim 1, wherein the bandgap ofsaid intrinsic layer of said second photovoltaic cell is between 1.45and 1.65 eV.
 6. The multijunction device of claim 1, wherein the bandgapof said intrinsic layer of said third photovoltaic cell is between 1.20and 1.45 eV.
 7. The multijunction device of claim 1, wherein the bandgapof said intrinsic layer of said first photovoltaic cell is between 1.7and 1.99 eV, the bandgap of said intrinsic layer of said secondphotovoltaic cell is between 1.45 and 1.65 eV, and the bandgap of saidintrinsic layer of said third photovoltaic cell is between 1.20 and 1.45eV.
 8. The multijunction device of claim 1, wherein said interface layeris doped with a dopant of the n-type to form a tunnel junction with saidp-type layer of said second photovoltaic cell and an ohmic contact withsaid n-type layer of said first photovoltaic cell.
 9. The multijunctiondevice of claim 8, wherein said n-type layer of said first photovoltaiccell and said p-type layer of said second photovoltaic cell furtherinclude an element selected from the group consisting of carbon,nitrogen, oxygen and fluorine; andsaid interface layer beingsubstantially free of carbon, nitrogen, oxygen, and fluorine.
 10. Themultijunction device of claim 1, wherein said interface layer is dopedwith a dopant of the p-type to form a tunnel junction with said n-typelayer of said first photovoltaic cell and an ohmic contact with saidp-type layer of said second photovoltaic cell.
 11. The multijunctiondevice of claim 1, further comprising a second interface layer of asemiconductor compound including silicon formed between said n-typelayer of said second photovoltaic cell and said p-type layer of saidthird photovoltaic cell, said second interface layer having a bandgapless than the bandgaps of said n-type layer of said second photovoltaiccell and said p-type layer of said third photovoltaic cell.
 12. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprisingap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer;an interface layer of a semiconductor compound including silicon formedon said n-type layer of said first photovoltaic cell, said interfacelayer having a bandgap less than the bandgap of said n-type layer ofsaid first photovoltaic cell; a second photovoltaic cell formed on saidinterface layer and comprisinga p-type layer formed on said interfacelayer and having a bandgap greater than the bandgap of said interfacelayer, an intrinsic layer formed on said p-type layer, and being made ofan amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said first photovoltaic cell, and an n-type layer formed onsaid intrinsic layer; a third photovoltaic cell formed on said secondphotovoltaic cell and comprisinga p-type layer, an intrinsic layer of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid second photovoltaic cell, formed on said p-type layer, and ann-type layer formed on said intrinsic layer; and a reflective electrodelayer formed on said n-type layer of said third photovoltaic cell,wherein said interface layer consists of first and second sublayersdoped with a dopant of the n-type, said first sublayer consisting of anamorphous hydrogenated silicon compound and said second sublayerconsisting of a microcrystalline hydrogenated silicon compound, saidfirst sublayer forming an ohmic contact with said n-type layer of saidfirst photovoltaic cell and said second sublayer forming a tunneljunction with said p-type layer of said second photovoltaic cell.
 13. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprisingap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer;an interface layer of a semiconductor compound including silicon formedon said n-type layer of said first photovoltaic cell, said interfacelayer having a bandgap less than the bandgap of said n-type layer ofsaid first photovoltaic cell; a second photovoltaic cell formed on saidinterface layer and comprisinga p-type layer formed on said interfacelayer and having a bandgap greater than the bandgap of said interfacelayer, an intrinsic layer formed on said p-type layer, and being made ofan amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said first photovoltaic cell, and an n-type layer formed onsaid intrinsic layer; a third photovoltaic cell formed on said secondphotovoltaic cell and comprisinga p-type layer, an intrinsic layer of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid second photovoltaic cell, formed on said p-type layer, and ann-type layer formed on said intrinsic layer; and a reflective electrodelayer formed on said n-type layer of said third photovoltaic cell,wherein said interface layer consists of a first layer and a secondlayer, said first layer being doped with a dopant of the n-type andforming an ohmic contact with said n-type layer of said firstphotovoltaic cell, said second layer being doped with a dopant of thep-type and forming an ohmic contact with said p-type layer of saidsecond photovoltaic cell, a tunnel junction being formed between saidfirst and second layers.
 14. The multijunction device of claim 13,wherein said first layer comprises a microcrystalline hydrogenatedsilicon compound and said second layer comprises an amorphoushydrogenated silicon compound.
 15. The multijunction device of claim 13,wherein said first and second layers each comprise an amorphoushydrogenated silicon compound.
 16. The multijunction device of claim 13,wherein said first and second layers each comprise a microcrystallinehydrogenated silicon compound.
 17. The multijunction device of claim 13,wherein said first layer comprises an amorphous hydrogenated siliconcompound and said second layer comprises a microcrystalline hydrogenatedsilicon compound.
 18. A multijunction photovoltaic device, comprising:atransparent substrate; a transparent electrode formed on saidtransparent substrate; a first photovoltaic cell formed on saidtransparent electrode and comprising;a p-type layer formed adjacent saidtransparent electrode, an intrinsic layer of an amorphous semiconductorcompound including silicon formed on said p-type layer, and an n-typelayer formed on said intrinsic layer; a second photovoltaic cell formedon said first photovoltaic cell and comprising,a p-type layer, anintrinsic layer formed on said p-type layer, having a thickness lessthan 3000 Å, and being made of an amorphous semiconductor compoundincluding silicon doped with germanium and having a bandgap less thanthe bandgap of said intrinsic layer of said first photovoltaic cell, andan n-type layer formed on said intrinsic layer; an interface layer of asemiconductor compound including silicon formed on said n-type layer ofsaid second photovoltaic cell, said interface layer having a bandgapless than the bandgap of said n-type layer of said second photovoltaiccell; a third photovoltaic cell formed on said interface layer andcomprising,a p-type layer having a bandgap greater than the bandgap ofsaid interface layer, an intrinsic layer of an amorphous semiconductorcompound including silicon doped with germanium and having a bandgapless than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell.
 19. Themultijunction device of claim 18, wherein the intrinsic layer of saidfirst photovoltaic cell has a thickness between 600 and 700 Å.
 20. Themultijunction device of claim 18, wherein the intrinsic layer of saidthird photovoltaic cell has a thickness between 1,500 and 2,500 Å. 21.The multijunction device of claim 18, wherein the bandgap of saidintrinsic layer of said first photovoltaic cell is between 1.7 and 1.99eV, the bandgap of said intrinsic layer of said second photovoltaic cellis between 1.45 and 1.65 eV, and the bandgap of said intrinsic layer ofsaid third photovoltaic cell is between 1.20 and 1.45 eV.
 22. Themultijunction device of claim 18, wherein said interface layer is dopedwith a dopant of the n-type to form a tunnel junction with said p-typelayer of said third photovoltaic cell and an ohmic contact with saidn-type layer of said second photovoltaic cell.
 23. The multijunctiondevice of claim 22, wherein said n-type layer of said secondphotovoltaic cell and said p-type layer of said third photovoltaic cellfurther including an element selected from the group consisting ofcarbon, nitrogen, oxygen and fluorine; andsaid interface layer beingsubstantially free of carbon, nitrogen, oxygen and fluorine.
 24. Themultijunction device of claim 18, wherein said interface layer is dopedwith a dopant of the p-type to form a tunnel junction with said n-typelayer of said second photovoltaic cell and an ohmic contact with saidp-type layer of said third photovoltaic cell.
 25. The multijunctiondevice of claim 18, further comprising a second interface layer of asemiconductor compound including silicon formed between said n-typelayer of said first photovoltaic cell and said p-type layer of saidsecond photovoltaic cell, said second interface layer having a bandgapless than the bandgaps of said n-type layer of said first photovoltaiccell and said p-type layer of said second photovoltaic cell.
 26. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprising:ap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer; asecond photovoltaic cell formed on said first photovoltaic cell andcomprising:a p-type layer, an intrinsic layer formed on said p-typelayer, and being made of an amorphous semiconductor compound includingsilicon doped with germanium and having a bandgap less than the bandgapof said intrinsic layer of said first photovoltaic cell, and an n-typelayer formed on said intrinsic layer; an interface layer of asemiconductor compound including silicon formed on said n-type layer ofsaid second photovoltaic cell, said interface layer having a bandgapless than the bandgap of said n-type layer of said second photovoltaiccell; a third photovoltaic cell formed on said interface layer andcomprising:a p-type layer having a bandgap greater than the bandgap ofsaid interface layer, an intrinsic layer of an amorphous semiconductorcompound including silicon doped with germanium and having a bandgapless than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell, wherein saidinterface layer consists of first and second sublayers doped with adopant of the n-type, said first sublayer consisting of an amorphoushydrogenated silicon compound and said second sublayer consisting of amicrocrystalline hydrogenated silicon compound, said first sublayerforming an ohmic contact with said n-type layer of said secondphotovoltaic cell and said second sublayer forming a tunnel junctionwith said p-type layer of said third photovoltaic cell.
 27. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprising:ap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer; asecond photovoltaic cell formed on said first photovoltaic cell andcomprising:a p-type layer, an intrinsic layer formed on said p-typelayer, and being made of an amorphous semiconductor compound includingsilicon doped with germanium and having a bandgap less than the bandgapof said intrinsic layer of said first photovoltaic cell, and an n-typelayer formed on said intrinsic layer; an interface layer of asemiconductor compound including silicon formed on said n-type layer ofsaid second photovoltaic cell, said interface layer having a bandgapless than the bandgap of said n-type layer of said second photovoltaiccell; a third photovoltaic cell formed on said interface layer andcomprising:a p-type layer having a bandgap greater than the bandgap ofsaid interface layer, an intrinsic layer of an amorphous semiconductorcompound including silicon doped with germanium and having a bandgapless than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell, wherein saidinterface layer consists of a first layer and a second layer, said firstlayer being doped with a dopant of the n-type and forming an ohmiccontact with said n-type layer of said second photovoltaic cell, saidsecond layer being doped with a dopant of the p-type and forming anohmic contact with said p-type layer of said third photovoltaic cell, atunnel junction being formed between said first and second layers. 28.The multijunction device of claim 27, wherein said first layer comprisesa microcrystalline hydrogenated silicon compound and said second layercomprises an amorphous hydrogenated silicon compound.
 29. Themultijunction device of claim 27, wherein said first and second layerseach comprise an amorphous hydrogenated silicon compound.
 30. Themultijunction device of claim 27, wherein said first and second layerseach comprise an microcrystalline hydrogenated silicon compound.
 31. Themultijunction device of claim 27, wherein said first layer comprises anamorphous hydrogenated silicon compound and said second layer comprisesa microcrystalline hydrogenated silicon compound.
 32. A multijunctionphotovoltaic device, comprising:a transparent substrate; a transparentelectrode formed on said transparent substrate; a first photovoltaiccell formed on said transparent electrode and comprising;a p-type layerformed adjacent said transparent electrode, an intrinsic layer of anamorphous semiconductor compound including silicon formed on said p-typelayer, and an n-type layer formed on said intrinsic layer; a firstinterface layer of a semiconductor compound including silicon formed onsaid n-type layer of said first photovoltaic cell, said first interfacelayer having a bandgap less than the bandgap of said n-type layer ofsaid first photovoltaic cell; a second photovoltaic cell formed on saidfirst interface layer and comprising,a p-type layer having a bandgapgreater than the bandgap of said first interface layer, an intrinsiclayer formed on said p-type layer, having a thickness less than 3000 Å,and being made of an amorphous semiconductor compound including silicondoped with germanium and having a bandgap less than the bandgap of saidintrinsic layer of said first photovoltaic cell, and an n-type layerformed on said intrinsic layer; a second interface layer of asemiconductor compound including silicon formed on said n-type layer ofsaid second photovoltaic cell, said second interface layer having abandgap less than the bandgap of said n-type layer of said secondphotovoltaic cell; a third photovoltaic cell formed on said secondinterface layer and comprising,a p-type layer having a bandgap greaterthan the bandgap of said second interface layer, an intrinsic layer ofan amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said second photovoltaic cell, formed on said p-type layer, andan n-type layer formed on said intrinsic layer; and a reflectiveelectrode layer formed on said n-type layer of said third photovoltaiccell.
 33. The multijunction device of claim 32, wherein the intrinsiclayer of said first photovoltaic cell has a thickness between 600 and700 Å.
 34. The multijunction device of claim 32, wherein the intrinsiclayer of said third photovoltaic cell has a thickness between 1,500 and2,500 Å.
 35. The multijunction device of claim 32, wherein the bandgapof said intrinsic layer of said first photovoltaic cell is between 1.7and 1.99 eV, the bandgap of said intrinsic layer of said secondphotovoltaic cell is between 1.45 and 1.65 eV, and the bandgap of saidintrinsic layer of said third photovoltaic cell is between 1.20 and 1.45eV.
 36. The multijunction device of claim 32, wherein said firstinterface layer is doped with a dopant of the n-type to form a tunneljunction with said p-type layer of said second photovoltaic cell and anohmic contact with said n-type layer of said first photovoltaic cell.37. The multijunction device of claim 36, wherein said n-type layer ofsaid first photovoltaic cell and said p-type layer of said secondphotovoltaic cell further including an element selected from the groupconsisting of carbon, nitrogen, oxygen and fluorine; andsaid firstinterface layer being substantially free of carbon, nitrogen, oxygen andfluorine.
 38. The multijunction device of claim 32, wherein said firstinterface layer is doped with a dopant of the p-type to form a tunneljunction with said n-type layer of said first photovoltaic cell and anohmic contact with said p-type layer of said second photovoltaic cell.39. The multijunction device of claim 32, wherein said second interfacelayer is doped with a dopant of the n-type to form a tunnel junctionwith said p-type layer of said third photovoltaic cell and an ohmiccontact with said n-type layer of said second photovoltaic cell.
 40. Themultijunction device of claim 39, wherein said n-type layer of saidsecond photovoltaic cell and said p-type layer of said thirdphotovoltaic cell further including an element selected from the groupconsisting of carbon, nitrogen, oxygen and fluorine; andsaid interfacelayer being substantially free of carbon, nitrogen, oxygen and fluorine.41. The multijunction device of claim 32, wherein said second interfacelayer is doped with a dopant of the p-type to form a tunnel junctionwith said n-type layer of said second photovoltaic cell and an ohmiccontact with said p-type layer of said third photovoltaic cell.
 42. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprising:ap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer; afirst interface layer of a semiconductor compound including siliconformed on said n-type layer of said first photovoltaic cell, said firstinterface layer having a bandgap less than the bandgap of said n-typelayer of said first photovoltaic cell; a second photovoltaic cell formedon said first interface layer and comprising:a p-type layer having abandgap greater than the bandgap of said first interface layer, anintrinsic layer formed on said p-type layer, and being made of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid first photovoltaic cell, and an n-type layer formed on saidintrinsic layer; a second interface layer of a semiconductor compoundincluding silicon formed on said n-type layer of said secondphotovoltaic cell, said second interface layer having a bandgap lessthan the bandgap of said n-type layer of said second photovoltaic cell;a third photovoltaic cell formed on said second interface layer andcomprising:a p-type layer having a bandgap greater than the bandgap ofsaid second interface layer, an intrinsic layer of an amorphoussemiconductor compound including silicon doped with germanium and havinga bandgap less than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell, wherein said firstinterface layer consists of first and second sublayers doped with adopant of the n-type, said first sublayer consisting of an amorphoushydrogenated silicon compound and said second sublayer consisting of amicrocrystalline hydrogenated silicon compound, said first sublayerforming an ohmic contact with said n-type layer of said firstphotovoltaic cell and said second sublayer forming a tunnel junctionwith said p-type layer of said second photovoltaic cell.
 43. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprising:ap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer; afirst interface layer of a semiconductor compound including siliconformed on said n-type layer of said first photovoltaic cell, said firstinterface layer having a bandgap less than the bandgap of said n-typelayer of said first photovoltaic cell; a second photovoltaic cell formedon said first interface layer and comprising:a p-type layer having abandgap greater than the bandgap of said first interface layer, anintrinsic layer formed on said p-type layer, and being made of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid first photovoltaic cell, and an n-type layer formed on saidintrinsic layer; a second interface layer of a semiconductor compoundincluding silicon formed on said n-type layer of said secondphotovoltaic cell, said second interface layer having a bandgap lessthan the bandgap of said n-type layer of said second photovoltaic cell;a third photovoltaic cell formed on said second interface layer andcomprising:a p-type layer having a bandgap greater than the bandgap ofsaid second interface layer, an intrinsic layer of an amorphoussemiconductor compound including silicon doped with germanium and havinga bandgap less than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell, wherein said firstinterface layer consists of a first layer and a second layer, said firstlayer being doped with a dopant of the n-type and forming an ohmiccontact with said n-type layer of said first photovoltaic cell, saidsecond layer being doped with a dopant of the p-type and forming anohmic contact with said p-type layer of said second photovoltaic cell, atunnel junction being formed between said first and second layers. 44.The multijunction device of claim 43, wherein said first layer comprisesa microcrystalline hydrogenated silicon compound and said second layercomprises an amorphous hydrogenated silicon compound.
 45. Themultijunction device of claim 43, wherein said first and second layerseach comprise an amorphous hydrogenated silicon compound.
 46. Themultijunction device of claim 43, wherein said first and second layerseach comprise an microcrystalline hydrogenated silicon compound.
 47. Themultijunction device of claim 43, wherein said first layer comprises anamorphous hydrogenated silicon compound and said second layer comprisesa microcrystalline hydrogenated silicon compound.
 48. A multijunctionphotovoltaic device comprising:a transparent substrate; a transparentelectrode formed on said transparent substrate; a first photovoltaiccell formed on said transparent electrode and comprising:a p-type layerformed adjacent said transparent electrode, an intrinsic layer of anamorphous semiconductor compound including silicon formed on said p-typelayer, and an n-type layer formed on said intrinsic layer; a firstinterface layer of a semiconductor compound including silicon formed onsaid n-type layer of said first photovoltaic cell, said first interfacelayer having a bandgap less than the bandgap of said n-type layer ofsaid first photovoltaic cell; a second photovoltaic cell formed on saidfirst interface layer and comprising:a p-type layer having a bandgapgreater than the bandgap of said first interface layer, an intrinsiclayer formed on said p-type layer, and being made of an amorphoussemiconductor compound including silicon doped with germanium and havinga bandgap less than the bandgap of said intrinsic layer of said firstphotovoltaic cell, and an n-type layer formed on said intrinsic layer; asecond interface layer of a semiconductor compound including siliconformed on said n-type layer of said second photovoltaic cell, saidsecond interface layer having a bandgap less than the bandgap of saidn-type layer of said second photovoltaic cell; a third photovoltaic cellformed on said second interface layer and comprising:a p-type layerhaving a bandgap greater than the bandgap of said second interfacelayer, an intrinsic layer of an amorphous semiconductor compoundincluding silicon doped with germanium and having a bandgap less thanthe bandgap of said intrinsic layer of said second photovoltaic cell,formed on said p-type layer, and an n-type layer formed on saidintrinsic layer; and a reflective electrode layer formed on said n-typelayer of said third photovoltaic cell, wherein said second interfacelayer consists of first and second sublayers doped with a dopant of then-type, said first sublayer consisting of an amorphous hydrogenatedsilicon compound and said second sublayer consisting of amicrocrystalline hydrogenated silicon compound, said first sublayerforming an ohmic contact with said n-type layer of said secondphotovoltaic cell and said second sublayer forming a tunnel junctionwith said p-type layer of said third photovoltaic cell.
 49. Amultijunction photovoltaic device comprising:a transparent substrate; atransparent electrode formed on said transparent substrate; a firstphotovoltaic cell formed on said transparent electrode and comprising:ap-type layer formed adjacent said transparent electrode, an intrinsiclayer of an amorphous semiconductor compound including silicon formed onsaid p-type layer, and an n-type layer formed on said intrinsic layer; afirst interface layer of a semiconductor compound including siliconformed on said n-type layer of said first photovoltaic cell, said firstinterface layer having a bandgap less than the bandgap of said n-typelayer of said first photovoltaic cell; a second photovoltaic cell formedon said first interface layer and comprising:a p-type layer having abandgap greater than the bandgap of said first interface layer, anintrinsic layer formed on said p-type layer, and being made of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid first photovoltaic cell, and an n-type layer formed on saidintrinsic layer; a second interface layer of a semiconductor compoundincluding silicon formed on said n-type layer of said secondphotovoltaic cell, said second interface layer having a bandgap lessthan the bandgap of said n-type layer of said second photovoltaic cell;a third photovoltaic cell formed on said second interface layer andcomprising:a p-type layer having a bandgap greater than the bandgap ofsaid second interface layer, an intrinsic layer of an amorphoussemiconductor compound including silicon doped with germanium and havinga bandgap less than the bandgap of said intrinsic layer of said secondphotovoltaic cell, formed on said p-type layer, and an n-type layerformed on said intrinsic layer; and a reflective electrode layer formedon said n-type layer of said third photovoltaic cell, wherein saidsecond interface layer consists of a first layer and a second layer,said first layer being doped with a dopant of the n-type and forming anohmic contact with said n-type layer of said second photovoltaic cell,said second layer being doped with a dopant of the p-type and forming anohmic contact with said p-type layer of said third photovoltaic cell, atunnel junction being formed between said first and second layers. 50.The multijunction device of claim 49, wherein said first layer comprisesa microcrystalline hydrogenated silicon compound and said second layercomprises an amorphous hydrogenated silicon compound.
 51. Themultijunction device of claim 49, wherein said first and second layerseach comprise an amorphous hydrogenated silicon compound.
 52. Themultijunction device of claim 49, wherein said first and second layerseach comprise an microcrystalline hydrogenated silicon compound.
 53. Themultijunction device of claim 49, wherein said first layer comprises anamorphous hydrogenated silicon compound and said second layer comprisesa microcrystalline hydrogenated silicon compound.
 54. A multijunctionphotovoltaic device, comprising:a transparent substrate; a transparentelectrode formed on said transparent substrate; a first photovoltaiccell formed on said transparent electrode and comprising;a p-type layerformed adjacent said transparent electrode, an intrinsic layer of anamorphous semiconductor compound including silicon formed on said p-typelayer, and an n-type layer formed on said intrinsic layer; a secondphotovoltaic cell formed on said first photovoltaic cell andcomprising,a p-type layer, an intrinsic layer formed on said p-typelayer, having a thickness less than 3000 Å, and being made of anamorphous semiconductor compound including silicon doped with germaniumand having a bandgap less than the bandgap of said intrinsic layer ofsaid first photovoltaic cell, and an n-type layer formed on saidintrinsic layer; a third photovoltaic cell formed on said secondphotovoltaic cell and comprising,a p-type layer, an intrinsic layer ofan amorphous semiconductor compound including silicon doped withgermanium and having a bandgap less than the bandgap of said intrinsiclayer of said second photovoltaic cell, formed on said p-type layer, andan n-type layer formed on said intrinsic layer; and a reflectiveelectrode layer formed on said n-type layer of said third photovoltaiccell.
 55. The multijunction device of claim 54, wherein the bandgap ofsaid intrinsic layer of said first photovoltaic cell is between 1.7 and1.99 eV, the bandgap of said intrinsic layer of said second photovoltaiccell is between 1.45 and 1.65 eV, and the bandgap of said intrinsiclayer of said third photovoltaic cell is between 1.20 and 1.45 eV. 56.The multijunction device of claim 54, further comprising an interfacelayer of a semiconductor compound including silicon formed between saidn-type layer of said first photovoltaic cell and said p-type layer ofsaid second photovoltaic cell, said interface layer having a bandgapless than the bandgaps of said n-type layer of said first photovoltaiccell and said p-type layer of said second photovoltaic cell.
 57. Themultijunction device of claim 59, further comprising a second interfacelayer of a semiconductor compound including silicon formed between saidn-type layer of said second photovoltaic cell and said p-type layer ofsaid third photovoltaic cell, said second interface layer having abandgap less than the bandgaps of said n-type layer of said secondphotovoltaic cell and said p-type layer of said third photovoltaic cell.58. The multijunction device of claim 54, further comprising aninterface layer of a semiconductor compound including silicon formedbetween said n-type layer of said second photovoltaic cell and saidp-type layer of said third photovoltaic cell, said interface layerhaving a bandgap less than the bandgaps of said n-type layer of saidsecond photovoltaic cell and said p-type layer of said thirdphotovoltaic cell.